Electronic devices typically incorporate low voltage DC power supplies to operate internal circuitry by providing a constant output voltage or current from a wide variety of input sources. Switching power converters are in common use to provide a voltage-regulated source of power, from battery, AC line and other sources.
Power converters operating from an AC line source (offline converters) typically require isolation between input and output in order to provide for the safety of users of electronic equipment in which the power supply is included or to which the power supply is connected. Transformer-coupled switching power converters are typically employed for this function. Regulation in a transformer-coupled power converter is typically provided by an isolated feedback path that couples a sensed representation of an output voltage from the output of the power converter to the primary side, where an input voltage (rectified line voltage for AC offline converters) is typically switched through a primary-side transformer winding by a pulse-width-modulator (PWM) controlled switch. The duty ratio of the switch is controlled in conformity with the sensed output voltage or current, providing regulation of the power converter output.
The isolated feedback signal provided from the secondary side of an offline converter is typically coupled to the primary side by an optoisolator or other circuit such as a signal transformer and a chopper circuit. The feedback circuit typically raises the cost and size of a power converter significantly and also lowers reliability and long-term stability, as opto-couplers change characteristics with age.
An alternative approach is often used for driving relatively static loads (light emitting diodes (LED), battery chargers). This approach uses an inherent property of a switching converter of a flyback type to deliver constant output power when operating in a discontinuous conduction mode (DCM). FIG. 1 depicts an example of such prior art transformer isolated LED driver circuit operating in the constant-power mode. The circuit includes an input voltage source 101, a flyback transformer 103 having a primary winding 111 and a secondary winding 112, a secondary rectifier diode 105, a smoothing capacitor 106, a switching transistor 102, a current sense resistor 104, a comparator 108 and a flip-flop circuit 109. In operation, a constant frequency CLOCK signal sets the flip-flop circuit 109. The transistor 102 periodically connects the primary winding 111 across the input voltage source 101. Comparator 108 monitors the current in the winding 111 by sensing voltage at the current sense resistor 104 and comparing it to a reference level REF. The comparator 108 resets the flip-flop 109 when the voltage at the resistor 104 exceeds REF. The transistor 102 turns off. The diode 105 conducts current from the secondary winding 112 to the LED load 107 and the smoothing capacitor 106. The diode 105 becomes reverse biased when the energy stored in the transformer 103 depletes fully and the current in the winding 112 reaches zero. Since the stored energy is proportional to the square of the current in the transformer 103 windings whose peak value is maintained constant, the circuit delivers constant power to the output load 107. For the same reason, the output power is also directly proportional to the CLOCK frequency. The output current of the flyback converter of FIG. 1 can be considered constant in the case of a fixed load. Therefore, this circuit could be used for supplying constant current output without the need for an opto-coupler feedback.
However, the circuit of FIG. 1 suffers some serious drawbacks. Firstly, the output current of this circuit is affected by variation in control circuit and load characteristics that are difficult to account for. The output current variation is caused by the inductance tolerances, switching frequency variation, tolerances and temperature drifting of the output load characteristics. Note that each 1% of the peak current error translates into a 2% output current error due to the quadratic output power dependence. A practical output current accuracy of a constant-power converter of FIG. 1 could be as poor as +/−50% or worse. Secondly, a constant-power converter must be tailored to a specific load and cannot be used as a universal constant current source. And thirdly, a constant-power converter is unprotected against output open-and short-circuit conditions. Indeed, the open-circuit output voltage and the short-circuit output current become uncontrollably high with the circuit of FIG. 1.
A simple circuit and method in accordance with the embodiments of the present invention yield a true universal constant-current source that is free of the above drawbacks.
Therefore, a need exists to provide a device and method to overcome the above problems.